A high voltage direct current (HVDC) system operates in such a manner that AC power generated by a power station is converted into DC power for power transmission and DC power is reconverted into AC power at a power receiving point for power supply. The HVDC system operates in a power transmission method capable of efficient and economical power transmission through voltage boosting, which is an advantage of an AC power transmission method, and capable of overcoming various disadvantages of AC power transmission. Recently, the HVDC system has applied to submarine cable power transmission, large-capacity long-distance power transmission, connection between AC systems, etc.
The HVDC system is controlled by various types of control devices. In particular, recently, a control device outputting control signals using a proportional-integral (PI) control method has been introduced. In the control device using the proportional-integral control method, the residual deviation between a target value and a controlled variable generated in a proportional controller is eliminated by an integral controller through a combination of a proportional controller (P) and an integral controller (I), thereby outputting a sophisticated control signal.
In the meantime, stable operation of the HVDC system is extremely important, and thus it is essential that related equipment, such as the control device for the HVDC system, etc. is operated by being configured in a dual manner. Generally, the most common dual controller used in the HVDC system operates in an active/stand-by manner. When an active controller in the middle of operation is abnormal, switching to a stand-by controller in a standby state is performed for operation. In this case, it takes a particular time for switching of the controller.
In order to solve the problem, a recent dual controller for the HVDC system operates in an active/active manner. In the active/active manner, two controllers in a dual manner operate simultaneously, and perform the same operation simultaneously in such a manner that while the HVDC system is controlled by a first controller, a second controller may operate any time by switching thereto.
However, in the dual controller for the HVDC system using the proportional-integral control method, the following problem occurs. When the HVDC system is operated by a control signal according to proportional-integral calculation of the first controller, the second controller performs the same proportional-integral calculation and outputs a control signal. Here, even though the same target value is simultaneously input to the first controller and the second controller, a phenomenon that error integral values of the integral controller are excessively accumulated occurs in actual operation due to various environmental and electrical factors, resulting in the divergence of the control value. In this state, when switching of the controller is performed, the HVDC system malfunctions due to the divergence of the control value.